1. Field of the Invention
This invention relates to a lowpass harmonic filter of the type used in output circuits of communications satellites. In particular, this invention relates to a waveguide lowpass filter that is capable of supporting three modes in a stopband.
2. Description of the Prior Art
Lowpass harmonic filters are used to augment isolation over a receive band of the filter and provide high attenuation for second and third harmonics of high level TWTAs.
As the number of satellites and frequency bands increases, it is becoming necessary to provide high isolation all the way from the receive band to the third harmonic in order to control spurious emission and to minimize interference with other satellite systems. It is known to have lowpass harmonic filters as discussed in a paper entitled "Tapered Corrugated Waveguide Low Pass Filters", by R. Levy, published in Institute of Electrical and Electronics Engineers Transaction on Microwave Theory and Technics, MTT-21, No. 8, dated August, 1973, pp. 526-532. These previous filters require low impedance sections in order to minimize spurious responses. Also, these filters have a relatively low power handling capability and can be relatively complex to fabricate and therefore expensive.
A waveguide lowpass filter having a single or double ridge structure is described in U.S. Pat. No. 3,949,327 dated April, 1976 and naming H. F. Chappell as inventor. This previous filter is capable of achieving a higher filter impedance than the corrugated waveguide structures but can only support one mode in both the passband and the stopband. The power handling capability of the filter described by Chappell is therefore relatively low in a vacuum environment.